The lawyer who defended Steven Avery and was one of the key figures in the Netflix series Making a Murderer says he is “relieved and gratified” by a judge's decision to overturn Brendan Dassey’s conviction for murder.

The lawyer who defended Steven Avery and was one of the key figures in the Netflix series Making a Murderer says he is “relieved and gratified” by a judge's decision to overturn Brendan Dassey’s conviction for murder.

Dassey, 26, could walk free from prison in 90 days if prosecutors decide against appealing the ruling delivered by a federal judge in Milwaukee on Friday.

Avery’s wrongful conviction for rape and attempted murder, exoneration, decision to sue the state and subsequent arrest and conviction for the murder of missing photographer Theresa Halbach was the subject of the popular Making a Murderer documentary.

His nephew Dassey was also convicted of the killing and sentenced to 41 years in prison. In his ruling, Judge William Duffin found his confession was “involuntary” and obtained by investigators making “false promises” to Dassey.

Dassey’s confession became one of the focuses of Making a Murderer. Dassey, who was 16 when Ms Halbach died, was a vulnerable teenager and the way in which his confession was extracted proved highly controversial.

In 2007, he was convicted of first-degree intentional homicide, second-degree sexual assault, and mutilation of a corpse by a jury in Wisconsin and told he could not apply for parole until 2048. He was 17 at the time of sentencing.

Dean Strang, Avery’s lawyer during his trial, said Dassey’s statements were “wholly unreliable and flatly wrong on essential details”. He told The Huffington Post: “Our federal courts are often the last protectors of our liberties and justice. We are thankful and proud that a federal court fulfilled its fundamental role for Brendan Dassey today. In doing so, this federal court served all Americans.”

Avery’s appeal is now being led by Kathleen Zellner, a renowned defence lawyer from Texas who is known for successfully appealing the convictions of 17 men.

The state will now decide whether to retry Dassey. If they decide against a retrial, he will be released. Netflix has confirmed a second series of Making a Murderer is being produced